Internet Engineering Task Force (IETF) T. Talpey
Request for Comments: 5667 Unaffiliated
Category: Standards Track B. Callaghan
ISSN: 2070-1721 Apple
January 2010
Network File System (NFS) Direct Data Placement
Abstract
This document defines the bindings of the various Network File System
(NFS) versions to the Remote Direct Memory Access (RDMA) operations
supported by the RPC/RDMA transport protocol. It describes the use
of direct data placement by means of server-initiated RDMA operations
into client-supplied buffers for implementations of NFS versions 2,
3, 4, and 4.1 over such an RDMA transport.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5667.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Talpey & Callaghan Standards Track [Page 1]

RFC 5667 NFS Direct Data Placement January 2010
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................21.1. Requirements Language ......................................22. Transfers from NFS Client to NFS Server .........................33. Transfers from NFS Server to NFS Client .........................34. NFS Versions 2 and 3 Mapping ....................................45. NFS Version 4 Mapping ...........................................65.1. NFS Version 4 Callbacks ....................................76. Port Usage Considerations .......................................87. Security Considerations .........................................98. Acknowledgments .................................................99. References ......................................................99.1. Normative References .......................................99.2. Informative References ....................................101. Introduction
The Remote Direct Memory Access (RDMA) Transport for Remote Procedure
Call (RPC) [RFC5666] allows an RPC client application to post buffers
in a Chunk list for specific arguments and results from an RPC call.
The RDMA transport header conveys this list of client buffer
addresses to the server where the application can associate them with
client data and use RDMA operations to transfer the results directly
to and from the posted buffers on the client. The client and server
must agree on a consistent mapping of posted buffers to RPC. This
document details the mapping for each version of the NFS protocol
[RFC1094] [RFC1813] [RFC3530] [RFC5661].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Talpey & Callaghan Standards Track [Page 2]

RFC 5667 NFS Direct Data Placement January 20102. Transfers from NFS Client to NFS Server
The RDMA Read list, in the RDMA transport header, allows an RPC
client to marshal RPC call data selectively. Large chunks of data,
such as the file data of an NFS WRITE request, MAY be referenced by
an RDMA Read list and be moved efficiently and directly placed by an
RDMA Read operation initiated by the server.
The process of identifying these chunks for the RDMA Read list can be
implemented entirely within the RPC layer. It is transparent to the
upper-level protocol, such as NFS. For instance, the file data
portion of an NFS WRITE request can be selected as an RDMA "chunk"
within the eXternal Data Representation (XDR) marshaling code of RPC
based on a size criterion, independently of the NFS protocol layer.
The XDR unmarshaling on the receiving system can identify the
correspondence between Read chunks and protocol elements via the XDR
position value encoded in the Read chunk entry.
RPC RDMA Read chunks are employed by this NFS mapping to convey
specific NFS data to the server in a manner that may be directly
placed. The following sections describe this mapping for versions of
the NFS protocol.
3. Transfers from NFS Server to NFS Client
The RDMA Write list, in the RDMA transport header, allows the client
to post one or more buffers into which the server will RDMA Write
designated result chunks directly. If the client sends a null Write
list, then results from the RPC call will be returned either as an
inline reply, as chunks in an RDMA Read list of server-posted
buffers, or in a client-posted reply buffer.
Each posted buffer in a Write list is represented as an array of
memory segments. This allows the client some flexibility in
submitting discontiguous memory segments into which the server will
scatter the result. Each segment is described by a triplet
consisting of the segment handle or steering tag (STag), segment
length, and memory address or offset.
struct xdr_rdma_segment {
uint32 handle; /* Registered memory handle */
uint32 length; /* Length of the chunk in bytes */
uint64 offset; /* Chunk virtual address or offset */
};
struct xdr_write_chunk {
struct xdr_rdma_segment target<>;
};
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RFC 5667 NFS Direct Data Placement January 2010
struct xdr_write_list {
struct xdr_write_chunk entry;
struct xdr_write_list *next;
};
The sum of the segment lengths yields the total size of the buffer,
which MUST be large enough to accept the result. If the buffer is
too small, the server MUST return an XDR encode error. The server
MUST return the result data for a posted buffer by progressively
filling its segments, perhaps leaving some trailing segments unfilled
or partially full if the size of the result is less than the total
size of the buffer segments.
The server returns the RDMA Write list to the client with the segment
length fields overwritten to indicate the amount of data RDMA written
to each segment. Results returned by direct placement MUST NOT be
returned by other methods, e.g., by Read chunk list or inline. If no
result data at all is returned for the element, the server places no
data in the buffer(s), but does return zeros in the segment length
fields corresponding to the result.
The RDMA Write list allows the client to provide multiple result
buffers -- each buffer maps to a specific result in the reply. The
NFS client and server implementations agree by specifying the mapping
of results to buffers for each RPC procedure. The following sections
describe this mapping for versions of the NFS protocol.
Through the use of RDMA Write lists in NFS requests, it is not
necessary to employ the RDMA Read lists in the NFS replies, as
described in the RPC/RDMA protocol. This enables more efficient
operation, by avoiding the need for the server to expose buffers for
RDMA, and also avoiding "RDMA_DONE" exchanges. Clients MAY
additionally employ RDMA Reply chunks to receive entire messages, as
described in [RFC5666].
4. NFS Versions 2 and 3 Mapping
A single RDMA Write list entry MAY be posted by the client to receive
either the opaque file data from a READ request or the pathname from
a READLINK request. The server MUST ignore a Write list for any
other NFS procedure, as well as any Write list entries beyond the
first in the list.
Similarly, a single RDMA Read list entry MAY be posted by the client
to supply the opaque file data for a WRITE request or the pathname
for a SYMLINK request. The server MUST ignore any Read list for
other NFS procedures, as well as additional Read list entries beyond
the first in the list.
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RFC 5667 NFS Direct Data Placement January 2010
Because there are no NFS version 2 or 3 requests that transfer bulk
data in both directions, it is not necessary to post requests
containing both Write and Read lists. Any unneeded Read or Write
lists are ignored by the server.
In the case where the outgoing request or expected incoming reply is
larger than the maximum size supported on the connection, it is
possible for the RPC layer to post the entire message or result in a
special "RDMA_NOMSG" message type that is transferred entirely by
RDMA. This is implemented in RPC, below NFS, and therefore has no
effect on the message contents.
Non-RDMA (inline) WRITE transfers MAY OPTIONALLY employ the
"RDMA_MSGP" padding method described in the RPC/RDMA protocol, if the
appropriate value for the server is known to the client. Padding
allows the opaque file data to arrive at the server in an aligned
fashion, which may improve server performance.
The NFS version 2 and 3 protocols are frequently limited in practice
to requests containing less than or equal to 8 kilobytes and 32
kilobytes of data, respectively. In these cases, it is often
practical to support basic operation without employing a
configuration exchange as discussed in [RFC5666]. The server MUST
post buffers large enough to receive the largest possible incoming
message (approximately 12 KB for NFS version 2, or 36 KB for NFS
version 3, would be vastly sufficient), and the client can post
buffers large enough to receive replies based on the "rsize" it is
using to the server, plus a fixed overhead for the RPC and NFS
headers. Because the server MUST NOT return data in excess of this
size, the client can be assured of the adequacy of its posted buffer
sizes.
Flow control is handled dynamically by the RPC RDMA protocol, and
write padding is OPTIONAL and therefore MAY remain unused.
Alternatively, if the server is administratively configured to values
appropriate for all its clients, the same assurance of
interoperability within the domain can be made.
The use of a configuration protocol with NFS v2 and v3 is therefore
OPTIONAL. Employing a configuration exchange may allow some
advantage to server resource management through accurately sizing
buffers, enabling the server to know exactly how many RDMA Reads may
be in progress at once on the client connection, and enabling client
write padding, which may be desirable for certain servers when RDMA
Read is impractical.
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RFC 5667 NFS Direct Data Placement January 20105. NFS Version 4 Mapping
This specification applies to the first minor version of NFS version
4 (NFSv4.0) and any subsequent minor versions that do not override
this mapping.
The Write list MUST be considered only for the COMPOUND procedure.
This procedure returns results from a sequence of operations. Only
the opaque file data from an NFS READ operation and the pathname from
a READLINK operation MUST utilize entries from the Write list.
If there is no Write list, i.e., the list is null, then any READ or
READLINK operations in the COMPOUND MUST return their data inline.
The NFSv4.0 client MUST ensure in this case that any result of its
READ and READLINK requests will fit within its receive buffers, in
order to avoid a resulting RDMA transport error upon transfer. The
server is not required to detect this.
The first entry in the Write list MUST be used by the first READ or
READLINK in the COMPOUND request. The next Write list entry is used
by the next READ or READLINK, and so on. If there are more READ or
READLINK operations than Write list entries, then any remaining
operations MUST return their results inline.
If a Write list entry is presented, then the corresponding READ or
READLINK MUST return its data via an RDMA Write to the buffer
indicated by the Write list entry. If the Write list entry has zero
RDMA segments, or if the total size of the segments is zero, then the
corresponding READ or READLINK operation MUST return its result
inline.
The following example shows an RDMA Write list with three posted
buffers A, B, and C. The designated operations in the compound
request, READ and READLINK, consume the posted buffers by writing
their results back to each buffer.
RDMA Write list:
A --> B --> C
Compound request:
PUTFH LOOKUP READ PUTFH LOOKUP READLINK PUTFH LOOKUP READ
| | |
v v v
A B C
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RFC 5667 NFS Direct Data Placement January 2010
If the client does not want to have the READLINK result returned
directly, then it provides a zero-length array of segment triplets
for buffer B or sets the values in the segment triplet for buffer B
to zeros so that the READLINK result MUST be returned inline.
The situation is similar for RDMA Read lists sent by the client and
applies to the NFSv4.0 WRITE and SYMLINK procedures as for v3.
Additionally, inline segments too large to fit in posted buffers MAY
be transferred in special "RDMA_NOMSG" messages.
Non-RDMA (inline) WRITE transfers MAY OPTIONALLY employ the
"RDMA_MSGP" padding method described in the RPC/RDMA protocol, if the
appropriate value for the server is known to the client. Padding
allows the opaque file data to arrive at the server in an aligned
fashion, which may improve server performance. In order to ensure
accurate alignment for all data, it is likely that the client will
restrict its use of OPTIONAL padding to COMPOUND requests containing
only a single WRITE operation.
Unlike NFS versions 2 and 3, the maximum size of an NFS version 4
COMPOUND is not bounded, even when RDMA chunks are in use. While it
might appear that a configuration protocol exchange (such as the one
described in [RFC5666]) would help, in fact the layering issues
involved in building COMPOUNDs by NFS make such a mechanism
unworkable.
However, typical NFS version 4 clients rarely issue such problematic
requests. In practice, they behave in much more predictable ways, in
fact most still support the traditional rsize/wsize mount parameters.
Therefore, most NFS version 4 clients function over RPC/RDMA in the
same way as NFS versions 2 and 3, operationally.
There are however advantages to allowing both client and server to
operate with prearranged size constraints, for example, use of the
sizes to better manage the server's response cache. An extension to
NFS version 4 supporting a more comprehensive exchange of upper-layer
parameters is part of [RFC5661].
5.1. NFS Version 4 Callbacks
The NFS version 4 protocols support server-initiated callbacks to
selected clients, in order to notify them of events such as recalled
delegations, etc. These callbacks present no particular issue to
being framed over RPC/RDMA, since such callbacks do not carry bulk
data such as NFS READ or NFS WRITE. They MAY be transmitted inline
via RDMA_MSG, or if the callback message or its reply overflow the
Talpey & Callaghan Standards Track [Page 7]

RFC 5667 NFS Direct Data Placement January 2010
negotiated buffer sizes for a callback connection, they MAY be
transferred via the RDMA_NOMSG method as described above for other
exchanges.
One special case is noteworthy: in NFS version 4.1, the callback
channel is optionally negotiated to be on the same connection as one
used for client requests. In this case, and because the transaction
ID (XID) is present in the RPC/RDMA header, the client MUST ascertain
whether the message is in fact an RPC REPLY, and therefore a reply to
a prior request and carrying its XID, before processing it as such.
By the same token, the server MUST ascertain whether an incoming
message on such a callback-eligible connection is an RPC CALL, before
optionally processing the XID.
In the callback case, the XID present in the RPC/RDMA header will
potentially have any value, which may (or may not) collide with an
XID used by the client for a previous or future request. The client
and server MUST inspect the RPC component of the message to determine
its potential disposition as either an RPC CALL or RPC REPLY, prior
to processing this XID, and MUST NOT reject or accept it without also
determining the proper context.
6. Port Usage Considerations
NFS use of direct data placement introduces a need for an additional
NFS port number assignment for networks that share traditional UDP
and TCP port spaces with RDMA services. The iWARP [RFC5041]
[RFC5040] protocol is such an example (InfiniBand is not).
NFS servers for versions 2 and 3 [RFC1094] [RFC1813] traditionally
listen for clients on UDP and TCP port 2049, and additionally, they
register these with the portmapper and/or rpcbind [RFC1833] service.
However, [RFC3530] requires NFS servers for version 4 to listen on
TCP port 2049, and they are not required to register.
An NFS version 2 or version 3 server supporting RPC/RDMA on such a
network and registering itself with the RPC portmapper MAY choose an
arbitrary port, or MAY use the alternative well-known port number for
its RPC/RDMA service. The chosen port MAY be registered with the RPC
portmapper under the netid assigned by the requirement in [RFC5666].
An NFS version 4 server supporting RPC/RDMA on such a network MUST
use the alternative well-known port number for its RPC/RDMA service.
Clients SHOULD connect to this well-known port without consulting the
RPC portmapper (as for NFSv4/TCP).
The port number assigned to an NFS service over an RPC/RDMA transport
is available from the IANA port registry [RFC3232].
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